I just bought both of these! I took the same size in both brands. They fit basically the same. same length, hips the same...BR waist gave me a very slightly bigger waist gap. The BR pants feel "higher quality," of course. But construction is decent on the ON version. There is a little side "vent" at the bottom of the legs on the BR version, which might make hemming more expensive (if you need to hem). I would say if you plan to wear them a whole lot, it might pay to keep the BR version, because I'm guessing they will last longer. Then again you could buy the ON version in 2 or 3 colors for the same price!
ETA: so I plan on exchanging my black Sloans for a gray pair (ON does not come in gray) and keeping my black ON's. The ON pant does feel a wee bit less substantial, but it is not really thin either (if it was I wouldn't wear them,gotta hide the cellulite. Ugh). Also, I sized up one size in both brands (bought a 2P instead of a 0P), because in my normal size both were too tight, especially on the hips.
